Method of separating trifluoroethylene from tetrafluoroethylene



Nov. 2, 1965 A. H. FAINBERG ETAL 3,215747 METHOD OF SEPARATINGTRIFLUOROETHYLENE Filed May 31, 1965 FROM TETRAFLUOROETHYLENE 4Sheets-Sheet l Nov. 2, 1965 A. FAINBERG ETAL 3215747 METHOD OFSEPARATING TRIFLOROETHYLENE FRM TETRAFLUORETHYLENE L BY MURRAYHAUPTSCHEIN ATTORNEY United States Patent O This invention relates tothe separation of relatively small amounts of trifluoroethylene tromtetrafluoroethylene by selective adsorption techniques.

In the production of tetrafluoroethylene by the high temperaturepyrolysis of fluoroform (CHF or difluorochlorornethane (CF HCI) the rawproduct gases often contain small amounts of trifluoroethylene,

(CF =CHF) The amount of trifluoroethylene produced may range from traceamounts up to about 2% by Weight (based on the Weight of the CF =CF +CF=CFH mixture).

In the polymerization of tetrafluoroethylene to high molecular polymerssuitable for molding into end products of high thermal stability andchemical inertness it has been found that it is of great importance toreduce the concentration of trifluoroethylene in the monomerictetrafluoroethylene to very low values such for example as to values ofless than 40 parts per million and preferably less than parts permillion and even as low as one part per million or less by weight oftrifluoroethylene. Concen trations of trifluoroethylene greater thanabout 10-40 p.pm. deleteriously affect the properties of the polymer, inparticular its thermal stab-ility. Polytetrafluoroethylene prepared frommonomer containing excessive amounts of trifluoroethylene tends toethermally degrade at the relatively high temperatures necessary forfabrication of the polymer into molded shapes resulting, e.g. in undueporosity, loss of tensile strength, loss of dielectric properties, etc.

Because of the closeness of their bolling points and their closesimilarity in other respects, the separation of trifluoroethylene fromtetrafluoroethylene down to such low residual trifluoroethyleneconcentrations is very diflicult by known methods. Thus, methods such asfractional distillation, -and liquid-liquid extraction and the like areirnpracticable for this purpose. Because of the high reactivity oftetrafluoroethylene, methods involving selective reaction of thetrifluoroethylene to a more easily separable compound are likewisedifficult or impossible t0 apply.

While it has been found that selective adsorption techniques using manycommon adsorbents such as silica gel, activated carbon and activatedalumina, are capable of making the required degree of separation, wehave discovered as will be shown below in more detail, that suchadsorbents have such a low capacity in the adsorption process that theiruse is not economically attractive. Thus, adsorbents of these types,even at relatively low concentrations of trifluoroethylene such asone-half percent, are able to process only 0.1 te 0.5 pound oftetrafluoroethylene per pound of adsorbent before exhausting theiradsorpti-on capacity.

In recent years, a relatively new class of adsorbents have corne intouse, partcularly in the field of selective 3,215,747 Patented Nov. 2,1965 hydrocarbon separation, consisting of crystalline metalaluminosilicates (often called zeolites) which in the dehydrated formhave a three dimensional network of aluminum and silicon oxides formingintracrystalline voids interconnected by pores of uniform size, oftenreferred to as zeolitic molecular sieves. The use of such adsorbents hasbeen previously suggested for the separation of vinyl fluoride tromvinylidene fluoride (see U.S. Patent 2,917,- 556 to Percival). In thisprevious work however it was found that the adsorption capacity of thesematerials in this separation was quite low. As will be shown in moredetail hereafter, even at relatively low concentrations of vinylfluoride, the capacity of the adsorbent was only about /2 pound of vinylfluoride-free vinylidene fluoride per pound of adsorbent. This is aboutthe same order of capacity that we have found for silica gel, activatedcarbon and activated alumina in the separation of trifluoroethylene. Itwas also found by Percival (U.S. Patent 2917,556) that the capacity ofthe adsorbent was essentially independent of the initial concentrationof vinyl fluoride, i.e virtually the same adsorption capacity, in termsof weight of pure vinylidene fluoride produced per weight of adsorbent,was obtained at both low and relatively high concentrations of vinylfluoride.

It has now been found in accordance with the present invention that, insurprising contrast to the behavior of the vinyl fluoride-vinylidenefluoride system as reported by Percival, the molecular sieve typecrystalline metal aluminosilicates have a very high capacity for theseparation of trifluoroethylene from tetrafluoroethylene (in many casesof the order of ten to thirty times greater than that for the separationof vinyl fluoride from vinylidene fluoride).

Furthermore, it has been found that such adsorbents are particularlyeflective for the selective adsorption of trifluoroethylene frommixtures of CF =CF and at relatively low initial concentrations ofCF,=CHF viz. at initial CF =CHF concentrations about 2% and less. Thus,it has been found that in contrast to the work reported by Percival forthe vinyl fluoride-vinylidene fluoride systern the capacity of molecularsieve type aluminosilicate adsorbents in thetrifluoroethylenetetrafluoroethylene system increases sharply atrelatively low concentrations of trifluoroethylene (below about 2%) thusmaking the use of such adsorbents particularly attractive in thesubstantially quantitative removal of 2% and less of trifluoroethylenefrorn tetrafluoroethylene.

The adsorbents used in the present invention may be described generallyas crystalline metal aluminosilicates which in the dehydrated form havea stable three-dimensional netwerk of SiO -and AIO, tetrahedra providingintracrylstalline voids which are interconnected by access openings orpores of uniform size. The eflective pore diameter should be at leastabout 5 A. (angstrom units). The electrovalence of the tetrahedracontaining aluminum is balanced by the inclusion in the crystal of acation, in particular, alkali metal or alkaline earth metal cations,especially sodium, potassium and calcium ions. The total void volumeafter dehydration is generally of the order of about 50%. Theseadsorbents are often referred to generally as zeolitic molecular sieves.

While there are a number of natural crystalline zeolites such aschabazite which have the above type of crystal structure and which rnayact as molecular sieves, most of these natural materials are unavailablein commercial quantities in sutficiently pure form, and in addition mosthave etective pore diameters which are too small for use in theinvention. For this reason the synthetic zeolitic molecular sieves -aremuch preferred for use in the present invention. These syntheticmaterials and their method of manufacture are described in detail inboth publications and in the patent literature. See for example I IershMolecular Sieves, Reinhold Publishing Corporation (1961) chapters 57;Breek et al., J.A.C.S. vol. 78 pp. 5963-5977; and U.S. Patents 2882,243and 2,882,244.

The type of synthetic zeolitic molecular sieves descrbed in U.S. Patents2882,243 and 2882,244 are particularly suitable for use in theinvention. Adsorbents of these types are commercially available e.g.from the Linde Division of Union Carbide Corporation under thedesignations eg. Molecular Sieve Types 5 A, X and 13 X.

The preferred sieves are those in which the interstitial metal cationsare alkali metal cations or those in which the original alkali metalcations have been replaced in whole or in part by alkaline earth metalcations. Particularly suitable are those in which the metal cations aresodium or calcium ions or both.

As pointed out previously the adsorbents used in the invention shouldhave pore openings (i.e. the openings givng access to theintracrystalline voids) with an effective pore diameter of at leastabout 5 A. The effective pore diameter refers to the crtical size of thesmallest molecule which will be admitted through the pores asdistinguished from the theoretical pore diameter formed by the frameworkof silica and alumina tetrahedra. Thus, Linde Molecular Sieve 5 A (fordetailed description see Exarnple II) has an effective pore diameter ofabout 5 A, will admit both tetrafluoroethylene and tri- ?luoroethylene,and is highly efective in their separation. Linde Molecular Sieve 4 A onthe other hand has an effective pore diameter of about 4 A and willadmit neither of those olefins and is ineffective in their separation.There is on the other hand no critical upper limit in the efiective poresize of the adsorbent. Thus, Linde Molecular Sieves 10 X and 13 X (tordetailed descrip tion see Examples IV and I respectively) havingeffective pore sizes of about 10 A and 13 A respectively, are bothhighly effective in separating trifluoroethylene fromtetrafluoroethylene in accordance with the inventon. As is apparent fromthe foregoing, the separation process of the invention does not dependupon the screenng of the two olefins according to size. Both enter thepores and are adsorbed on the surfaces of the intracrystalline voids asindicated by the fact that both trifluoroethylene andtetrafiuoroethylene are taken up by sieves having effective porediameters large enough to admit them in amounts of the order of 10 to40% by weight of the adsorbent. Such a large take up could only beaccounted Eor by the adsorption of the olefins on the large internalsurfaces provided by the intracrystalline voids. The area of theinternal surfaces are generally of the order of 600-800 square metersper gram in contrast to an external area of only 1 to 3 square metersper gram. Rather than depending upon screenng by size, the separation ofthe two olefins according to the invention depends upon a differentaladsorption effect with the tnifluoroethylene neing more stronglyadsorbed than the tetrafluoroethylene. In this connection it is mostsurprising that two naterials as closely related as tetrafluoroethyleneand :rifluoroethylene should exhibit such a large difierential1dsorption (as demonstrated by the high capacity of the adsorbent forremoving trifluoroethylene) particularly 2vhen the vinylfluoride-vinylidene fluoride system exhibts such a low diferentialadsorption effect as demon ;trated by the low capacity of the sameadsorbents for ;eparating vinyl fluoride from vinylidene fluoride.

The zeolitic molecular sieves are, of course, used in the activatedanhydrous form, de. the crystal water has been driven off leavingintracrystalline voids. Suitable sieves are available in activated,essentially anhydrous (no adsorbed water) condition, containing onlyabout 1 weight percent adsorbed air. If water is adsorbed prior to useby exposure, for example, to a humid atmosphere, the adsorbed water canbe readily removed by heating the sieves at a temperature of the orderof 350 C. While evacuating to low pressure-or sweeping with a purge gas,e.g. air or nitrogen.

The zeolitic molecular sieves will be employed in any convenent physicalform such as a powder or in the form of pellets. The pellet form ispreferred from the stand point of avoiding undue pressure drops throughthe system, for uniformity of flow, and case of handling. Generally,pellets ranging from 5 to 4" in size will be found satisfactory. Thepellets are usually prepared by formulating the zeolite with about 20%by weight of an inert binder and then compressing the mixture into pel-Iets.

The capacity of an adsorbent in the separaton of a two component mixtureA+B where B is the more strongly adsorbed component and is present inrelatively minor amount, can be conveniently expressed in terms of theweight of pure A (i.e. A free from B) that can be obtained per unitweight of evacuated adsorbent at the time of initial breakthrough. Byevacuated adsorbent is meant adsorbent that contains no adsorbedcomponent. The capaoity value in these terms is designated by the symbolX. The time of initial breakthrough (t is the time which elapses afterstarting the flow of the A+B mixture through a column of the adsorbentuntil the first trace of component B is detected in the column effluent.The value of t is readily determined experimentally by monitoring thecomposition of the effluent to determine the time of appearance of thefirst trace of B. The valve of X is calculated from the followingrelationship.

l i i w where u. is the rate of feed of A+B to the exacuated adsorbentin grams per minute; 1 is the time of: initial breakthrough in minutes;w is the total weight in grams of A+B which is adsorbed on w grams ofadsorbent at t and where w is the weight in grams of the evacuatedadsorbent. The value of w can be determined experimentally by measuringthe weight gain of the evacuated adsorbent at 21.

The capacity of the adsorbent in the separation of a two componentmixture A+B where B is the more strongly adsorbed component and ispresent in relatively minor amount, canalso be expressed in terms of theweight of effluent obtained per unit weight of adsorbent at the time of50% breakthrough. The capacity value in these terms is designated by thesymbol Y. The time of 50% breakthrough is the time which elapses afterstarting the flow of the A+B mixture through a column of the adsorbentuntil the concentration of B in the effluent is 50% of its concentrationin the feed mixture of A+B. The value of t is readily determinedexperimentally by monitoring the compostion of the effluent to determinewhen the concentration of B has risen to 50% of its concentration in thefeed composition. The value of Y is calculated from the followingrelatonship:

where U, t and w are as defined above and where w is the total weight ingrams of A+B which is adsorbed on w grams of adsorbent at I The value ofw can be determined experimentally by measuring the weight gain of theevacuated adsorbent at t The advantage of expressing the capacity interms of Y (capacity based on 50% breakthrough, (t rather than in termsof X (capacity at initial breakthrough (t is that the value of Y moreclosely represents the maximum capacity value that can be attained inactual practice. Furthermore, the value of Y is virtually independent offeed velocity, column length, ratio of the column length to diameter,adsorbent particle size, adsorbent particle sze distribution andadsorbent packing density.

As pointed out above, the value of Y closely approximates the maximumpurification capacity attainable (in terms of weight of pure component Aobtainable per unit weight of adsorbent). This results from the factthat the capacity at 1 for most systems is essentially equivalent to theequilibrium purification capacity Y which is defined as the weight ofpure component A obtainable per unit weight of adsorbent when theadsorbent is operated to the point at which it is unable to adsorbfurther quantities of component B, that is until the composition of theefliuent from the adsorbent is the same as the composition of the feed.At this point the adsorbent is in equilibrium with the feed. Therelationship of Y and Y can be better understood by reference to FIGURE1, where the concentraton of component B in the effluent from theadsorbent, expressed as the percent of its concentration in the feed, isplotted against time. From time t (ie. the start of feed to theactivated adsorbent) to t (time of initial breakthrough) theconcentration of component B in the efliuent is zero. During this timepure A is recovered. At t the first trace of B appears in the eflfluentand from t to t the concentration of B increases at the rate shown bycurve 1 until at t the concentration of B in the efliuent is 100% of itsconcentration in the feed. At this point the adsorbent is in equilibriumwith the feed gas and no further separation of B from A will occur. At tthe concentration B in the eflluent is 50% of its concentration in thefeed. The amount of component B which has appeared in the eflluent at tis proportional to the shaded area D while the residual capacity of theadsorbent to remove component B from the mixture A+B at t isproportional to the shaded area E. If curve 1, defining the rate atwhich the concentration of B increases with respect to time, issymmetrical, as it is in FIGURE 1, then elapsed time t to t is the sameas t to t and the area D is equal to area E. In these circumstances, theamount of. B which has appeared in the eflluent at (represented by areaD) is the same as the amount of B that can still be adsorbed by theadsorbent at t (represented by area E). Thus, when curve 1 issymmetrical the maximum capacity, viz. the equilibrium capacity Y of theadsorbent (in terms of weight of pure A obtainable per weight ofadsorbent) becomes equal to Y. Since in most cases curve 1 isapproximately symmetrical in shape, Y can be generally taken forpractical purposes as representing the maximum capacity (i.e.equilibrium capacity) of the adsorbent. Even in cases where curve 1 issomewhat a symmetrical the value of Y will generally closely approximateY EXAMPLES I T0 IV The following Examples I t0 IV illustrate theseparation of trifluoroethylene from tetrafluoroethylene by selectiveadsorption on various types of zeolitic molecular sieves according tothe invention and demonstrate the high purification capacitiesattainable. In each exarnple the adsorbent in the form of pelletscontaining about 20% by weight of an inert bonding material is loadedinto a vertical tube with a 40 cm. length of packed section and aninside diameter of 2 om. (lengthzdiameter ratio of 20:1). The evacuated(no adsorbed component) weight of the adsorbent in each case is shown inTable I. In Example I, the adsorbent is a zeolitic molecular sieve ofthe type described in U.S. Patent 2,882,244 supplied by the LindeDivision of Union Carbide Corporation under the designation MolecularSieve Type 13 X. It has the general chemical formula 6 It has the Xcrystal structure which is cubic, a =2495 angstroms, space group and ischaracterized by a 3-dimensional netwerk of A10 and Si0 tetrahedra whichafter removal of crystal water form mutually connected intra-crystallinevoids accessible through openings (pores) which will admit moleculeswith critical dimensions up to 9 A. The void volume is 51 vol. percent.

T able l Example I II III IV Type of adsorbent 13X 5A AW-500 10XEvacuated weignt of adsorbent, gms.-. 69.3 76. 3 73. 1 69. 4 WeightPercent CF2=CHF in feed 0. 45 0. 41 0. 41 0. 43 Mess fiow of feed,grams/mnute 2. 2. 94 3. 00 2. 92 Superfieial linear velocity, ft.lsec.-0.129 0.128 0.131 0.128 Initial breakthrough time, t, min. 286 6 99 50%breakthrough time, t5, min 357 247 153 148 Weight of OF3=CF2+OF2=CFH onadsorbent after t, gms 18. 2 16. 8 10. 4 17. 2 Purifieation. capaeity X(i.e. at ti) 11. 9 5. 8 2. 5 3. 9 Purification capacity Y (i.e. at t5)14. 9 9. 3 6. 3 6. 0

with an average volume of voids of about 0.38 cubic centimeters pergram. It has an internal surface area of 650-800 square meters per gramand an external surface area of 1 to 3 square meters per gram.

In Example II, the adsorbent is a zeolitic sieve of the type describedin United States Patent 2882,243 supplied by the Linde Division of UnionCarbide Company under the designation Molecular Sieve Type 5 A. Thisadsorb ent is prepared from Linde molecular sieve type 4 A by exchanging(through ion exchange) about 75% of the sodium ions of Linde MolecularSieve Type 4 A for calcium ions. Linde Molecular Sieve Type 4 A, whichis also of the type described in United States Patent 2,882243, has thegeneral chemical formula Molecular sieve type 5 A has the A crystalstructure which is cubic, a =12.32 A., space group characterized by athree dimensional network of A104 and 810 tetrahedra which after removalof crystal water form mutually connected intra-crystalline voidsaccessible through openings (pores) which will admit molecules withcritical dimensions up to about 5 A. in diameter. The void volume isabout 45% with an average volume of voids of 0.27 cubic centimeters pergram. It has an internal surface area of 650 to 800 square meters pergram and external surface area of 1 to 3 square meters per gram.

In Example 111, the adsorbent is a zeolitic molecular sieve supplied bythe Linde Division of Union Carbide Company under the designationMolecular Sieve Type AW-500. This sieve is similar to the adsorbentsused in Examples I, 11 and IV, being characterized by a threedimensional network of A10 and Si0.; tetrahedra which after removal ofcrystal water form mutually connected intra-crystalline voids accessiblethrough openings (pores) which will admit molecules with criticaldimensions of slightly less than about 5 A. It is resistant to acidattack and thus is specially adapted for use in acidic environments.

In Example IV, the adsorbent is a zeolitic molecular sieve of the typedescribed in United States Patent 2,882,- 244 supplied by the LindeDivision of Union Carbide Company under the designation Molecular SieveType 10 X. This is prepared from Linde Molecular Sieve Type 13 X,described above, by exchanging (through ion exchange) about 75% of thesodium ions of molecular sieve type 13 X for calcium ions. Type 10 X hasthe name crystal structure as Type 13 X and approximately he same vidvolume, but the pores are somewhat small- :r and will admit moleculesonly with critical dimensions lp to about 8 A. It has an internal andexternal surface .ICEL similar to that for Type 13 X.

In each of the examples, a mixture of tetrafluoroethylme andtrifluoroethylene containing a concentration of rifluoroethylene asshown in Table I is introduced into he bottom of the vertical column ofadsorbent at ambient emperature (25 C.). The pressure in the system is1pproximately atmospheric (15 p.s.ia). The adsorbents tre employed asreceived in their air-loaded condition, the 1dsorbed air beingimmediately displaced when the flow )f the mixture is started. Themixture is introduced into he column at a constant mass flow rate andsuperficial inear velocty as shown in Table I for each of the four uns.

The composition of the eflluent gas from the column is :ontinuouslymonitored by passage through a gas Sam )ling valve of a gaschromatograph. By sampling and 1nalyzing the eflluent at frequentintervals, the time of nitial breakthrough (t and the time of 50%breakhrough (t is determined for each run. The product Trom theadsorbent is collected in a liquid nitrogen cooled 'eceiver.

At the outset, while the air is being desorbed and re- )laced bytetrafluoroethylene, the temperature of the :olumn in the zone wheredesorption and replacement is aking place increases, and the warm zonetravels up the :olumn. After displacement of the air, the column cools 0ambient temperature (25 C.) and continues at this emperature during theremainder of the operation. The teat of displacement of thetetrafluoroethylene by the rifluoroethylene is too small to exert anysignificant emperature elect because of the relatively small percenttgesof trifluoroethylene in the feed.

In each case, the time (t,) of initial breakthrough of F CHF ismeasured. This is taken at the time when he concentration of CF =CHF inthe effluent exceeds hirty parts per million. The time of 50%breakthrough t is also measured. The values of w (total amount )f CF =CFand CFF-CFH adsorbed at t and the alue of w (weight of CF =CF and CF=CFH on he adsorbent at t are measured by the weight gain of headsorbent. In practice, it is not necessary to measu.re

is of the order of 3 to 12 grams of trifluoroethylene-treetetrafluoroethylene per gram of adsorbent.

The very high purification capacities attainable in accordance with theinvention becorne apparent when compared to the purification capacitiesobtainable with other commonly used adsorbents such as silica gels,activated carbons and activated alumina. In the following examples(Examples A, B, C, D and E), a series of tests were made underconditions similar to these used in Examples I to IV to determine thecapacity of silica gel, activated carbon, and activated alumina for theremoval of trifluoroethylene from tetrafluoroethylene by selectiveadsorption. In each example, the adsorbent in the form of small pelletsor granules is loaded into a vertical tube with a 40 cm. length ofpacked section and an inside diameter of 2 cm. (lengthzdiarneter ratioof 20:1). The evacuted (no adsorbed component) weight of the adsorbentis shown in Table II. In Example A the adsorbent is a granular silicagel having a total surface area of the order of 800 square meters pergram. In Example B, the adsorbent is a silica gel in the form of beadshaving a surface area of the order of 600 square meters per gram. InExample C, the adsorbent is an activated carbon obtained by thedestructive distillaton of bituminous coal and having a total surfacearea of the order of 1000 to 1200 square meters per gram and a porevolume of about 0.8 cubic centimeters per gram. In Example D, theadsorbent is an activated carbon obtained by the destructivedistillation of coconut shells having a surface area similar to that ofExample C. In Example E, the adsorbent is activated alumina consistingover 99% of alumina and low in sodium, iron and silica, and having asurface area of about 230 square meters per gram. A feed mixture oftetrafluoroethylene containing a small amount of trifluoroethylene isintroduced into the bottom of the column at the mass flow rate andsuperficial linear velocty shown in Table II. The time at initialbreakthrough and at breakthrough are measured in the manner previouslydescribed. The weight of tetrafluoroethylene plus trifluoroethyleneadsorbed after t, is also determined in the manner described above. Thepurification capacity X (at t and Y (at 1 is shown in Table II. As isapparent, the purification capacities obtainable with the use of thezeolitic molecular sieve type adsorbents is 20 to 100 times greater thanwith these other types of adsorbents.

T able II Example A B 0 D E Type of adsorbent Silica gel Siliea gelActivated Activated Activated Mass flow of feed, grams/rninuteSuperficial linear velocty, ft./see

Initial broakthrough time t minnfe 50% breakthrough time tro, minnh=Weight of CF;= CF +CF CI-IF on adsorbent after t, gms

Purillcation capacity Y (i.e. at 1150) Purification capacity X (i.e. att) 0.1

hese weight gains precisely at t and t After t there s only a very smallchange in the total weight of As mentioned previously, an importantfeature of the invention is the high capacity of the zeolitic molecularsieves for the removal of low initial concentrations oftrifluoroethylene, viz. about 2% by weight and less, tromtetrafluoroethylene. The manner in which the capacity of several typesof zeolitic molecular sieves varies with respect to the initialconcentration of trifluoroethylene in mixtures of CF =CF and CF =CFHcontaining from 0 to 6% of trifluoroethylene is shown in FIGURES 2 and3. In FIGURE 2, curves 2, 3 and 4 show respectively the manner in whichcapacity varies with initial concentration of trifluoroethylene formolecular sieves Type 10 X, Type 5 A and Type 13 X, where capacity isexpressed in terms of Y (i.e. at t In FIGURE 3,

=xpressed in terms of X (capacity at initial breakthrough) curves 5, 6and 7 show respectively the manner in which 9 capacity varies withrespect to inital trifluoroethylene concentration for molecular sievesType 10 X, Type 5 A and Type 13 X, where capacity is expressed in termsof X (i.e. at t As is apparent from these curves, at initialtrifluoroethylene concentrations over about 2% by weight, the capacityvaries only siightly with respect to trifluoroethylene concentration,while in the case of trifluoroethyleue concentrations of less than 2%capacity rises rapidly. The enhanced capacity of the zeolitic sieves forthe rernoval of trifluoroethylene frorn tetrafluoroethylene at 10Winitial concentrations of trifluoroethylene of 2% and 1ess makes theprocess of the invention of particular value in this concentrationregion. Thus, the process of the invention is of great value in theultra-purfication of tetrafluoroethylene by the removal of 2% or less,and particularly for the removal of 1% or less, of trifluoroethylenetrom mixtures of CF =CF and CF =CFH down to 10W values of the order of20 parts per million (by weight) and preferably to less than 10 partsper million of residuai trifluoroethylene.

The enhanced capacity of the zeolitic sieves for the removal oftrifluoroethylene frorn tetrafluoroethylene at 10W trfluoroethyleneooncerrtrations is in surprisng contrast to the behavior displayed byvinylidene fluoridevinyl fluoride mixtures using the same type ofadsorbents, as reported by Percival in United States Patent 2,917,556.Reference is made to 'Irab1e II which shows the capacities obtained bythe use of zeolitic molecular sieves for the renroval of vinyl fluoridefnom vinylidene fluoride in Examples 1 to 3 reported in United StatesPatent 2,917,- 556. The purification capacities obtained in theremaining examples of the patent are indicated to be substantially thesame as these reported for Exampies 1 to 3. The purification capacitiesare expressed in term of X (i.e. at t snce this value can be calculatedfrom the data reported by Percival, and is directly comparable to thevalue shown herein fior the CF =CF +CF =CFH system. It is apparent firstof all that the purification capacites determinedfrom Percivals data aremuch 1ess at all impurity concentrations than those obtained for the CF=CF +CF =CFH system in accondance with the invention. In the area ofparticular interest (i.e., impurity concentratons of about 2% and less)the differenccs between the respective purification capacities of thetwo systems are greatly different. For example, at an impurity level ofabout 0.5% the capacities obtained for the CF =CF +CF =CFH system inaccordance with the invention are more than twenty t1imes greater thanfor the CF =CH +CFH=CH system. In the second place, it can be seen thatthe capacity of the zeolitic sieves is essentially constant for the CF=CH -ICFH=CH system irrespective of the initial concentration of vinylfluo- T able III REMOVAL OF VINYL FLUO RIDE FROM VINYLIDENE FLUO RIDEUSING ZEOLITE MOLECULAR SIEVES (DATA FROM U.S. PATENT 2917,566W. O.PERCIVAL) Percivais Exampie N 1 2 3 Type of Adsorbent 13X 13X 13XEvacuated weght of adsorbent, grns. 742. 5 708 742. 5 Concentration ofCH CHF in ieed, mol percent 6. 4 0. 61 23. 2 Concentration of CH2=CHF infeed, wt. percent. 4. 7 0.44

Superficial iinear velocity, feet per sec. Mass flow of feed, grams perminute. Initiai breakthrough time, t, minutes. Weight of OH =CF +CH =CHFadsorbed after ride in the mixture. Enhanced oapacity a-t relatively 1owconcentrations of 2% and less is not obtained as in the case of thepresent invention. This is illustrated clearly in FIGURE 3 where curve100 shows the manner in 10 which the pun'fioation capacity X inPercivals systeni CF =CH +CFH=CH vares with concentration at vinylfluoride concentratioug between about 0.5 110 6%. As may be =seen thecapacity is virtually unchanged over this range.

In general, the adsorption separation process of the invention may becarred out at temperatures ranging from about 50 C 10 +50 C. andpreferably from 30 to +30 C. It has been fourrd that the cap;acity ofthe zeolitic molecular sieves increases at lower temperatures and apartcularly preferred range of operatng temperature is from 20" to [20C.

Examples V to VII nclusive which are summarized in Table IV, show thevariation of purification capacity with respect to temperature for aType 10 X molecular sieve at temperatures from +20 to 26 C. Theseexamples Were carrierd out in the same manner and using the sameequipment as in previous Examples I to IV inclusive. As -may be seen,the purification :capacity Y (i.e., at t of the sieve increases from 6.0gr-ams of trifluoroethylenefree tetrafluoroethylene per gram ofadsorbent at [25 C. to a capacity of 16.0 at 26 C. The variation ofcapacity with respect 'o temperature for the Type 10 X sieve is showngraphically in FIGURE 4 where curve 8 shows the variation of capacitywith temperature at constant pressure at temperatures of 30 10 +30 C.

Aside from the fall ofl in capacity as the temperature increases, it isalso desirable to avoid temperatures over about +30 C. from thestandpoint of minimizing the tendency of tetrafluoroethylene topolymerize on the zeo litic sieves.

Table 1 V Example V VI VII Type of adsoxbent 10X 10X 10X ernp C 25 0 26Pressure, p.s.ia 15 15 15 Evacuated weight of adsorbent, grams. 69. 469. 1 69.9 Weight percent of CF2=CHF in feed 0.43 0. 42 0.42 Mass flowof feed, grams per minute 2. 92 2. 3. 03 Superfieiai linea.r velocity,ft./sec 0. 128 0. 114 0. Initial breakthrough time, t, minut 99 181 32250% breakthrough time, t5 minutes 148 253 37 Weight of CF2=CF2+CF2=CFHon adsorbent aiter t gms 17. 2 19.0 21. 0 Purification capacity X (i.e.at t) 3.9 7. 2 13. 7 Purification eapaeity Y (i.e. at t o) 6.0 10. 216.0

The pressure employed during separation is not critical in the sense ofdeterminiug whether or not the separaton will take place. Thus,subatmospheric pressures, normal pressures and super-atmospheric pressures may be ernployed. Super-atmospheric pressures greater than 300p.s.ina. (-.pounds per square inch absolute) are preferably avoidedbecause of the greater tendency of tetrafluoroethylene to polymerze onthe sieves at such pressures. Although sub-atmospheric pressures as 10W,for example as 5 p.s.i.a. may be employed if desired, it is =generallymore convenient to operate at normal or moderate superatmosphericpressures. Aflthough it has been found that the capacity of the sieve isnot greatly pressure dependent, there is some capacity iucrease as thepressure increases frorn atmospheric to about 50 p.s.i.a. Sirrce thetetrafluoroethylene will ordinarily be handled under pressure it willgenera1ly be convenient t o operate at moderate super-atmosphericpressures.

Examples VIII 00 X inciusive, which are summarized in 'Fable V, show thevariation in capacity at constant temperature of about 25 C. as thepressure increases from 15 to 75 p.s.a. for a 10 X type molecular sieve.These examples were carried out using the same equip ment and the sameprocedures as described for the previous Examples I to IV. As may beseen, the purification capacity Y (i.e., at t increased from 6.0 to7.15. FIG- URE 5 shows graphically the effect of incre.asing pressure onpurificaton capacity in the range of 10 to 75 p.s.ia. for a Type 10 Xmolecular sieve at constant temperature. Curve 9 showing thisrelatonship illustrates Table V Example VIII IX X Type of adsorbent 10X10X 10X Temp., C 25 24 25 Pressure, n s i 2 15 29 75 Evacuated wt. ofabsorbent, gms 69. 4 69. 1 82.3

Weight percent of CF =CHF in feed Mass flow of feed, grams/minuteSuperficial linear velocity, ft./see- 0. 067 Initial breakthrough time,in, minutes 99 109 131 60% breakthrough time, t5o, minutes 148 160 206Weight: of CF =CF +OF =CFH on adsorben alter t grams 17. 2 18. 5 24.8Purification capacity X (i.e. at t;) 3. 9 4. 4 4. 4 Purificationcapacity Y (i.e. at tw) 6.0 6. 6 7. 15

sorbent to low pressures and elevated temperatures whereupon both of theadsorbed components i.e. tetrafluoroethylene and trfluoroethylene aredesorbed. Elevated temperatures of from 150 to 350 C. and preferablyfrom 180 to 300 C. and reduced pressures of one millimeter of mercury orless will generally be used. Where the concentraton of trfluoroethylenein the feed is relatively low i.e of the order of 2% or less, theadsorbent after saturation will contan a relatively high proportion oftetrafluoroethylene. The adsorbed tetrafluoroethylene may be recoveredseparately, with a desorption of only a small proportion of thetrfluoroethylene adsorbed on the sieve, by a stepwise regenerationprocedure in which the saturated sieve is first subjected to relativelylow temperatures and reduced pressures to remove most of the lessstrongly adsorbed tetrafluoroethylene and only small amounts of the morestrongly adsorbed trfluoroethylene, after which the temperature israised and/or the pressure decreased to remove trfluoroethylene. Thetetrafluoroethylene-rich eflluent from the first stage containing onlysmall proportions of trfluoroethylene may then be recycled to a freshsieve, and in ths way the tetrafluoroethylne content recovered. Forexample, a X type molecular sieve after reachng saturation when used forthe removal of approximately 0.4% trfluoroethylene fromtetrafluoroethylene will release 98% of the adsorbed tetrafluoroethyleneand only about 8% by weight of the adsorbed trfluoroethylene whensubjected to evacuation to a pressure of 0.2 mm. Hg at a temperature of25 C. The remainder of the trfluoroethylene may then be removed in asecond stage by heating to a temperature of 180 C. at.0.2 mm. Hg forabout 1 hour. The sieve is then cooled and subsequently pre-loaded withpure tetra fluoroethylene to the operating temperature and pressure. Thesieve, thus regenerated, is highly effective and may be reused for thetreatment of further quantities of impure tetrafluoroethylene (i.e.containing trfluoroethylene) to yield tetrafluoroethylene contaning lessthan 2 p.p.m. of trfluoroethylene.

Other methods that may be used for regeneration of the zeolitic sievesinclude, for example, selective displacement of the tetrafluoroethyleneby passing carbon dioxide through the sieve at room temperature. Thedisplacement of the tetrafluoroethylene by the C0 is essentiallyquantitative, and only small quanttes of trfluoroethylene are removed,such that the tetrafluoroethylene thus recovered is suitable for recycleto a fresh sieve for recovery of the tetrafluoroethylene content.Following the CO treatment, the adsorbed CO and trfluoroethylene maythen be removed by purging with an inert gas such as ntrogen, preferablyat an elevated temperature such as 150 C. to 300 C.

Still another regeneration method that may be employed is direct purgingwith an inert gas such as nitrogen at elevated temperatures of e.g. 150C. to 300 C. The tetrafluoroethylene is quantitatively removed and thetri fluoroethylene may be removed down to the required level. Thislatter method however has the disadvantage that the tetrafluoroethyleneis dificult to recover from its admixture with nitrogen.

The Examples XI to X111 nclusive which are summarizedin Table VIillustrate the successive regeneration and reuse of a Type 5 A molecularsieve for the purificaton of tetrafluoroethylene containing about 0.4percent trfluoroethylene by weight. In Example XI a new sieve wasemployed while in Exarnple XII the same sieve was employed afterregeneration by evacuaton to a prcssure of about 0.25 mm. Hg and atemperature of about 250 C. for about minutes. In Example XIII the samesieve was again employed after a second regeneration un der similarconditions. The equpment and procedures employed in these examples werethe same as those used in Examples I to IV. As may be seen, thepurification capacity dropped only slightly on the first regenerationand remained essentially constant after the second regeneration.

Table VI Example XI XII XIII Type ofadsorbent 5A 5A 5A Number ofregenerations- Temp.0

Pnrification eapacty X (i.e. at t) Puri.fication capacity Y (i.e. at t 1New sieve. 2 First regeneration. Second regeneration.

In large scale commercial operations t will be generally desirable toemploy multiple columns of adsorbent arranged in series and suitablymanifolded so that they may be successvcly operated to full capacity andsuccessively regenerated. An example of a suitable arrangement is shownin FIGURE 6 wherein reference numerals 10, 11 and 12 refer to thecolumns of adsorbent. The raw feed (i.e.trifluoroethylene-c0ntaining-tetrafluoroethyl ene) is fed to the columnsthrough line 13 and manifold 14. Manifold 14 is connected to each of thethree columns by branch lnes 15, 16 and 17 controlled by valves 15a, 16aand 17a respectively. The purified efluent from the adsorbent is removedby manifold 18 connected to the top of the columns by branch lines 20,21 and 22 controlled by valves 20a, 21a and 22a respectively. Purfiedtetrafluoroethylene is withdrawn from the system by line 19 for anydesired use.

T0 regenerate the adsorbent a.fter saturation, the adsorbent columns 10,11 and 12 are provided with heating and coolng means (not shown) bywhich the adsorbent can be heated to the desired regenerationtemperature and subsequently cooled to operating temperature. Manifold23 is provided at the top of the columns and is connected to the columnsby branch lines 24, 25 and 26 controlled by valves 24a, 25a and 26arespectively for wthdrawng desorbed material from the adsorbent duringthe regeneration cycle. Vacuum pump 27 is provided to reduce the columnsto the desired regeneration pressure. Desorbed material may be removedfrom the system by line 28 controlled by valve 28a, while portions of tmay be recycled to line 13 by line 29 controlled by valve 29a.Compressor 2% is provided to compress the recycled material to thepressure in line 13. At comple- 13 tion of regeneration, the adsorbentcolumns are cooled down to operating temperature and loaded with puretetrafluoroethylene to operating pressure.

The top of each column is connected to manifold 33 at the bottom of thecolumns by branch lines 30, 31 and 32 which are controlled by valves30a, 31a and 32a respectively. Manifold 33 is in turn connected bybranch lines 34, 35 and 36, controlled by valves 34a, 35a and 36arespectively, to the bottom of the columns. By means of lines 30, 31 and32, manifold 33, and lines 34, 35 and 36, the columns of adsorbent maybe selectively interconnected with one another in series as will bedescribed below.

At the beginning of the operation, When all the adsorbent is fresh, theraw feed is introduced through line 13 into column 1 through line 15 andvalve 15a While purified eflluent is taken ofi at the top of columnthrough line 20 and valve 20a and removed from the system throughmanifold 18 and line 19. During this period of operation all othervalves are closed.

At some time before column 10 has reached its initial breakthrough point(i.e. before the concentration of trifluoroethylene in the eflluent hasreached some predetermined limit such as 10 parts per million) column 11is placed in series with column 10 by closing valve 20a and openingvalves 30a, 3511 and 21a. Flow through column 10 is continued until theadsorbent therein has become fully saturated (i.e. the concentration oftrifluoroethylene in its efiluent has reached the concentration in theraw feed). At this point column 10 is ready for regeneration. Valves a,a, a and a are closed While valve 16a is opened to permit the raw feedto pass directly into the bottom of column 11. Column 10 is then placedin the regeneration cycle by heating the column, opening valves 24a and28a and placing the column under vacuum through vacuum pump 27. Atcompletion of regeneration, valve 29a is closed, and the column iscooled to operating temperature under vacuum, valve 24a is closed, andthe adsorbent is loaded with pure tetrafluoroethylene to operatingtemperatures and pressure through line 20 and valve 20a. Whileregeneration of column 10 is proceeding, the purification proceeds incolumn 11 until it approaches initial breakthrough, at which time it isplaced in series with column 12 by closing valve 21a and opening valves31a, 36a and 22a. When column 11 has become fully saturated, it too isshut down and placed in the regeneration cycle in the manner describedfor column 10.

When column 12 approaches its point of initial breakthrough, it is thenplaced in series with column 10 by closing valve 22a and opening valves32a, 34a and 20a. Column 10 by this time, has been regenerated and isready for further purification duty. When column 12 is completelysaturated, it too is shut down for regeneration in the manner previouslydescribed.

By thus successively interconnecting the columns in series the entirepurification capacity of each of the columns may be -utilized While atthe same time obtaining an eflluent of any desired purity. Any number ofcolumns, of course, may be interconnected with one another in the mannershown to undergo successive purification and regeneration cycles.

During the regeneration, the first portions of the effluent,particularly where the regeneration is conducted in stages with aninitial low temperature stage, will be rich in the more easily desorbedtetrafluoroethylene. This tetrafluoroethylene-rich,trifuoroethylene-lean portion may, if desired, be recycled to the systemfor recovery of the desorbed tetrafluoroethylene by line 29. In thesecond stage of the regeneration where the efllfiluent is principallytrifluoroethylene, the regeneration eflluent may be withdrawn from thesystem by line 28.

The optimum rate of feed of the tetrafluoroethylenetrifiuoroethylenemixture to the adsorbent is readily de 14 termined empirically.Generally, superficial linear velocities in the range of from 0.01 to1.0 feet per second will be found satisfactory.

Under some conditions, such as operation at atmospheric pressures andreduced temperatures, e.g. 0 C. it will be possible to conduct theseparation in the absence of a polymerization inhibitor for thetetrafluoroethylene. Under other conditions, on the other hand, such asoperation at superatmospheric pressures, and/ or at somewhat highertemperatures, it will be desirable to conduct the separation in thepresence of an inh.ibitor such as dipentene or Terpene B or otherterpenes of the type, for example, described in United States Patent2,407405. Since many of the molecular sieves tend to adsorb suchinhibitors at least to some extent, in order to insure the presence ofthe inhibitor throughout the column of adsorbent, it is preferable topretreat the ads-orbent with inhibitor, such as by tumbling pellets ofthe adsorbent in an atmosphere of inhibitor vapor prior to changing thepellets into the adsorbent column. In this manner, the pellets areuniformly treated with the inhibitor such that inhibitor is presentthroughout the column. The presence of the inhibitor has no substantialeffect upon the capacity of the adsorbent for the removal oftrifluoroethylene from tetrafluoroethylene.

The separation of the trifluoroethylene from tetrafluoroethylene willoccur in the presence of other components that are often found in thepyrolysis product produced by the pyrolysis of fluoroform or CF HCl suchas hexafluoroethane, C F octafluoropropane, C F fluoroform, CHFpentafluoroethane, C HF perfluorobutyne-2, CF3CECCF3, perfluoropropene,CF CCF perfluorocyclobutane, c-C.;F; perfluorobutene-Z While thecapacity for the removal of trifluoroethylene may be somewhat decreasedby some of these impurities, its removal will nevertheless proceed intheir presence.

It is to be understood that the foregoing specific embodiments andillustrative exarnples are given by way of illustration and that theinvention is not limited thereto.

We claim:

1. A method for separating trifluoroethylene trom tetrafluoroethylenewhich comprises contacting a mixture of tetrafluoroethylene andtrifluoroethylene containing not more than 2% by weight oftrifluoroethylene with a cnystalline metal aluminosilicate having in thedehydrated form a stable, three-dimensional network of SO. and A10tetrahedra containing interstitial metal cations, said network providingintracrystalline voids intercom nected by pores having an effectivediameter of at least about 5 A. thereby preferentially adsorbing saidtrifluoroethylene on said aluminosilicate, and recoveringtetrafluoroethylene essentially free from trifluoroethylene.

2. A method in accordance with claim 1 in which the separation iscarried out at a temperature of from 30 to +30 C.

3. A method in accordance with claim 1 in which the separation iscarried out at a pressure of from 15 to 300 p.s.1.a.

4. A method for separating trifluoroethylene from tetrafluoroethylenewhich comprises contacting a mixture of tetrafluoroethylene andtrifluoroethylene containing not more than 2% by weight oftrifluoroethylene with a crystalline metal aluminosilicate having in thedehydrated form a stable, three-dimensional network of SO and A10;tetrahedra containing interstital metal cations selected from the classconsisting of alkali metal and alkaline earth metal cations, saidnetwerk providing intracrystalline voids interconnected by pores havingan eirective diameter of at least about 5 A. thereby preferentiallyadsorbing said trifluoroethylene on said alumino- 1 5 ilcate, andrecoverng tetrafiuoroethylene essentally free from trifluoroethylene.

5. A method in accordance with claim 4 in Which said ;eparaton is carredout at a temperature of from 30 10 -|30 C.

6. A meth0d in accordance with claim 4 in which said separation iscarred out at a pressure of from 15 to 30 p.s.1.a.

References Cited by the Examiner- UNITED STATES PATENTS 2917,556 12/59Percival 260653.3

LEON ZITVER, Prmwny Examiner.

DANIEL D. HORWITZ, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No3,215,747 November 2, 1965 Arnold Harold Painberg et al.

It is hereby certfied that error appears in the above numberedpatentrequirng correcton and that the said Letters Patent should read ascorreoted below.

Column l, line 36, for "ethermally" read thermally column 2, line 18,after "roethylene" insert from tetrafluoroethylene column 4, line 37,for "valve" read value column 9, line 28, for "Table II" read Table IIIsame column 9, in the headng to Table III, line 3 thereof, fox"2,917,566 read 2,917,556 column 10, line 16, for "+20" read +25 column11, Table V, under the heading "Example", line5 5 and 1() thereof, for

"C1 each occurrence, read CF column 12, Table VI, first CQlumn, line 6thereof, for "CF read CF column 13, line 16, for "column l" read columnl() Signed and sealed this 19th day of July 1966.

(SEAL) Attest;

ERNEST W SWIDER EWARD J. BRENNER Attestng Officer Commissioner ofPatents

1. A METHOD FOR SEPARATING TRIFLUOROETHYLENE FROM TETRAFLUOROETHYLENEWHICH COMPRISES CONTACTING A MIXTURE OF TETRAFLUOROETHYLENE ANDTRIFLUOROETHYLENE CONTAINING NOT MORE THAN 2% BY WEIGHT OFTRIFLUOROETHYLENE WITH A CRYSTALLINE METAL ALUMINOSILICATE HAVING IN THEDEHYDRATED FORM A STABEL, THREE-DIMENSIONAL NETWORD OF SIO4 AND ALO4TETRAHEDRA CONTAINING INTERSTITIAL METAL CATIONS, SAID NETWORK PROVIDINGINTRACRYSTALLINE VOIDS INTERCONNECTED BY PORES HAVING AN EFFECTIVEDIAMETER OF AT LEAST ABOUT 5 A., THEREBY PREFERENTIALLY ADSORBINT SAIDTRIFLUOROETHYLENE ON SAID ALUMINOSILICATE, AND RECOVERINGTETRAFLUOROETHYLENE ESSENTIALLY FREE FROM TRIFLUOROETHYLENE.